High density boron nitride matrix composites

ABSTRACT

A method of manufacturing a composite material comprises forming a mixture comprising a plurality of fibers and a borazine oligomer; subjecting the mixture to a first heating, for 12 hours to 56 hours; and subjecting the mixture to a second heating. The temperature of the first heating is 60° C. to 80° C., and the pressure during the first heating is at least 0.5 MPa, the temperature of the second heating is at most 400° C., and the greatest pressure of the second heating is at least 15 MPa.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract NumbersDMR-9523735 and DMI-0127834 awarded by the National Science Foundation(NSF). The government has certain rights in the invention.

BACKGROUND

Materials which do not deteriorate mechanically and continue to functionin extreme conditions of temperature, friction and wear, are desirableand necessary in a wide range of modern applications. The materials ofaircraft brakes experience some of the harshest service conditions seenin any application. The material experiences high friction andtemperatures while the aircraft is taxiing and landing. In addition, allaircraft brake materials are required to withstand the extremeconditions of rejected take-off (RTO) trials.

One class of materials used for aircraft brakes is composites of carbonfibers reinforcing a carbon matrix (C/C composites). These materials aredescribed in J. Economy, H. Jung and T. Gogeva, Carbon, Vol. 30, No. 1,pp 81-85 (1991).

C/C composites are considerably stronger and lighter than steel. Suchmaterials increase in strength with increasing heat treatment and resistthermal shock caused by rapid temperature changes. However, they sufferfrom a number of drawbacks including poor oxidation resistance, highlyvariable wear rate and coefficient of friction, and costlymanufacturing. The lack of high temperature stability requires anexpensive supplementary processing step to coat the non-frictionalsurfaces with an oxidation barrier. In addition, fabrication is verytime consuming (1-2 months) due to the long periods required forchemical vapor infiltration of the carbon matrix. Thus, the fabricationis a slow, expensive process. The carbon matrix is usually introducedamong the carbon fibers by liquid impregnation and charring of organicmaterials. In some applications, chemical vapor deposition is used as afinal step in processing. The steps in the processes are repetitive andcan take months to complete. These materials are also only moderatelyoxidation resistant, thus requiring the addition of an expensiveoxidation barrier coating to the non-frictional surfaces. Furthermore,they have a highly variable coefficient of friction, especially in thepresence of water, which causes variable brake feel to pilots (nicknamed“morning sickness”).

Carbon fibers reinforcing a boron nitride matrix (C/BN composites) havethe potential to overcome some of the shortcomings of C/C composites.These materials are prepared by first polymerizing borazine (B₃N₃H₆), toyield an oligomer of appropriate viscosity that is used as a boronnitride precursor for impregnating the carbon fibers. The impregnatedfibers are then heated under pressure, yielding a solid boron nitridematrix reinforced by the carbon fibers. The preparation time for suchcomposites requires days, as opposed to the months needed for C/Ccomposites, with concomitant cost savings (U.S. Pat. No. 5,399,377).However, due to low density, the resulting C/BN composites do not haveacceptable heat capacity and thermal conductivity values to substitutefor C/C composites in aircraft brakes. The preparation typically yieldsa composite with a density in the range of 1.38 g/cc to 1.43 g/cc, andwith multiple impregnations (up to six) the density increases up to 1.61g/cc. Additional impregnations do not appear to effectively increase thedensity of the composites.

Consequently, C/BN composites have higher oxidation resistance than C/Ccomposites, but do not have the desired density of about 1.8 g/cc whichis expected to be necessary for the materials to have the heat capacityand thermal conductivity needed for good braking.

The performance of C/C composites may also be improved by the additionof a boron nitride matrix, yielding carbon fiber composite materialswith a boron nitride layer, or C/C/BN composites. These materials areprepared by immersing lower density C/C composites in borazine oligomersand subjecting the system to the same procedure used for C/BN composites(U.S. Pat. No. 5,399,377). The boron nitride coating renders the productmore oxidation resistant than C/C composites. Nevertheless, the densityof C/C/BN composites is still below the level required for aircraftbrakes applications, and their wear rate is too high once the thin BNcoating is worn away (Brian Fabio, M. S. Thesis, The University ofIllinois at Urbana-Champaign).

SUMMARY

In a first aspect, the present invention is a method of manufacturing acomposite material comprising forming a mixture comprising a pluralityof fibers and a borazine oligomer; subjecting the mixture to a firstheating, for 12 hours to 56 hours; and subjecting the mixture to asecond heating. The temperature of the first heating is 60° C. to 80°C., and the pressure during the first heating is at least 0.5 MPa, thetemperature of the second heating is at most 400° C., and the greatestpressure of the second heating is at least 15 MPa.

In a second aspect, the present invention is a composite materialcomprising carbon fibers in a boron nitride matrix. The compositematerial has a density of at least 1.62 g/cc.

In a third aspect, the present invention is a composite materialcomprising carbon fibers in a boron nitride matrix. The compositematerial has a wear rate of at most 0.4 mg/m at an energy level of 100kJ/kg to 1100 kJ/kg, and a coefficient of friction of at least 0.22 atan energy level of 100 kJ/kg to 1200 kJ/kg.

In a fourth aspect, the present invention is a method of manufacturing acomposite material comprising boron nitride, comprising forming amixture comprising a preform and a borazine oligomer; subjecting themixture to a first heating, for 12 hours to 56 hours; and subjecting themixture to a second heating. The temperature of the first heating is 60°C. to 80° C., and the pressure of the first heating is at least 0.5 MPa,and the temperature of the second heating is at most 400° C., and thegreatest pressure of the second heating is at least 15 MPa.

In a fifth aspect, the present invention is a composite materialcomprising a 3D needled carbon fiber preform impregnated with boronnitride having a density of at least 1.63 g/cc.

In a sixth aspect, the present invention is a composite material,comprising CVI-infiltrated carbon fiber preform impregnated with boronnitride having a density of at least 1.62 g/ cc.

In a seventh aspect, the present invention is a composite materialcomprising a 3D needled carbon fiber preform impregnated with boronnitride having a wear rate of at most 0.05 mg/m at an energy level of100 kJ/kg to 1000 kJ/kg, and a coefficient of friction of at least 0.12at an energy level of 100 kJ/kg to 900 kJ/kg.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 illustrates a thermogravimetric analysis (TGA) experimentperformed on a 1:1 mixture of carbon fibers and borazine.

FIG. 2 is the X-ray diffraction scan of the C/BN composites of theinvention, showing that the d-spacing of BN is 3.36 A.

FIG. 3 is a plot of the wear rate for traditional C/C composites,traditional C/C/BN composites, and the high density C/BN composites ofthe invention.

FIG. 4 is a plot of the coefficient of friction (COF) for traditionalC/C composites, traditional C/C/BN composites, and the high density C/BNcomposites of the invention.

FIG. 5 is a plot of the wear rate at 160 psi for the high density C/BNmaterial of the invention and for traditional lower density C/BNmaterials.

FIG. 6 is a plot of the coefficient of friction (COF) at 160 psi for thehigh density C/BN material of the invention and for traditional, lowdensity C/BN materials.

FIG. 7 is the X-ray diffraction scan of the 3D C/C/BN composites of theinvention, showing that the d-spacing of BN is 3.38 A.

FIG. 8 is a plot of the wear rate, at a pressure of 0.25 MPa, oftraditional 3D C/C composites, the 3D C/BN composites of the invention,and of the 3D C/C/BN composites of the invention.

FIG. 9 is a plot of the wear rate, at a pressure of 0.5 MPa, oftraditional 3D C/C composites, the 3D C/BN composites of the invention,and of the 3D C/C/BN composites of the invention.

FIG. 10 is a plot of the coefficient of friction (COF), at a pressure of0.25 MPa, of traditional 3D C/C composites, the 3D C/BN composites ofthe invention, and of the 3D C/C/BN composites of the invention.

FIG. 11 is a plot of the coefficient of friction (COF), at a pressure of0.5 MPa, of traditional 3D C/C composites, the 3D C/BN composites of theinvention, and of the 3D C/C/BN composites of the invention.

DETAILED DESCRIPTION

The present invention provides a new type of C/BN composites withdensities averaging 1.75 g/cc, and methods for their fabrication. TheseC/BN composites were found to outperform C/C composites in testing, andtheir density renders them a viable substitute for C/C composites inapplications of friction and wear. The invention is based on thediscovery that higher density C/BN composites can be formed when carbonfibers impregnated with borazine oligomers are heated under a pressureexceeding 0.5 MPa, and the pressure is applied before the temperaturereaches 400° C.

These high density C/BN composites are prepared by first heating theborazine monomer in a pressure vessel, with occasional venting, at atemperature of 60° C. to 80° C., more preferably 65° C. to 75° C., and,most preferably, 68° C. to 72° C. The borazine reacts with itself toyield oligomers while releasing hydrogen as a primary by-product, andthe heating is applied until a viscosity of 500 cP to 2500 cP isattained, usually within 24 to 48 hours. The oligomers are used toimpregnate carbon fibers, and the resulting system is heated to 60° C.to 80° C., more preferably 65° C. to 75° C., and most preferably, 68° C.to 72° C., under a pressure ranging from 0.5 to 8 MPa, more preferably 1to 5 MPa, and most preferably 2.2 to 4.4 MPa, for preferably 12 to 48,hours to induce further oligomerization of the borazine withoutformation of voids.

The composite is then subject to increasing temperature and pressureunder an inert atmosphere. The heating promotes further borazinepolymerization while the pressure is controlled to achieve a highdensity product. The temperature is increased to 300° C. to 400° C. at arate of 0.25° C./min to 3° C./min, more preferably to 325° C. to 375° C.at a rate of 0.75° C./min to 1.25° C./min, yet more preferably tobetween 340° C. to 360° C. at a rate of 0.9° C./min to 1.1° C./min. Thepressure is ramped to a final pressure of 14 MPa to 30 MPa, morepreferably from 18 MPa to 26 MPa, and most preferably, 21 MPa to 23 MPa.The composite is then preferably held at the final temperature andpressure for an additional 10 to 30 hours, more preferably 16 to 24hours.

The resulting composite is preferably heated at 5° C./min to 15° C./minto a final pyrolysis temperature of 1100° C. to 1500° C., for 1 to 3hours, and then cooled, more preferably heated at 10° C./min to a finalpyrolysis temperature of 1150° C. to 1250° C., for about 2 hours. Thisprocess is optionally carried out two or three times to maximize thedensity of the product.

The process described above was also applied to 3D needled carbon fiberpreforms. The hydrogen and other by-products appear to easily escapefrom the preform, yielding a new type of high density 3D C/BN materialwith as little as a single impregnation. In addition, it was found thatoptimal wear properties were obtained when the 3D carbon preform wasfirst subjected to carbon CVI to a density of ˜1.3 g/cc and thenimpregnated with borazine oligomer and subjected to the process above,yielding a new higher density 3D C/C/BN material.

Objects of various shapes may be fabricated by using the materials ofthe invention in resin transfer molding processes (RTM). In RTM, a fiberpreform, with the shape of the desired object, is loaded into a moldwithout the borazine. Alternatively, the preform may be bent into thedesired shape by the walls of the mold. The mold is closed, and borazineor the borazine oligomer as described above is injected or transferredinto the mold and impregnates the preform. The resulting system is thensubjected to the same temperature and pressure treatment as describedabove, yielding objects with increased densities and complex shapes infaster processing times.

EXAMPLES

1) Thermogravimetric Analysis of the C/BN Composite Production Process

One possible explanation for the higher density of the materials of theinvention may be that the pressure prevents the hydrogen evolving fromthe transformation of the borazine into boron nitride from formingbubbles that would lead to a porous, lower density structure. Toinvestigate this possibility, a series of thermogravimetric analysis(TGA) experiments were done using a 1:1 mixture of carbon fibers andborazine.

FIG. 1 is a representative TGA showing that the majority of the hydrogenand other by-products evolved from the borazine below 150° C. Thisindicated that applying pressure would be most effective below thistemperature for two reasons: (1) after attaining 150° C. the viscositywould become too high and the sample less compressible; (2) this is theregion where most of the hydrogen by-product is being produced, a highpressure at this temperature could help prevent the introduction ofpores into the matrix structure. In general, the system should bemaintained under pressure at a temperature of at most 150° C. untilabout 80% of the weight loss due to borazine polymerization hasoccurred. However, pressure during the higher temperature processing,150° C. to about 350° C., is still essential in order to keep theevolving hydrogen from creating porosity in the structure.

2) Preparation of High Density C/BN Composites

Borazine monomer was oligomerized in an inert atmosphere at 70° C. for36 hours, until an oligomer of viscosity of 500 to 2500 cP was attained.In a dry box, chopped carbon fibers were placed into a two inch diametermold and a measured amount of the borazine oligomer was added. Graphitefoil was then placed on top of the sample (as a releasing agent)followed by a tightly fitting disk of TEFLON® (DuPont, Wilmington, Del.)to prevent potential leakage of borazine. To provide extra mechanicalsupport to the TEFLON® a precisely machined brass disk was placed on topof the TEFLON®. A steel plunger was then placed on top of the brass andthe entire assembly transferred into a controlled atmosphere hot press.The mold and the sample were then heated to 70° C. and held isothermallyfor an additional 48 hours under pressure. Pressure was applied whilethe temperature was increased to 350° C. at a rate of 1° C./min underdry nitrogen. The composite was then held at 350° C. for 20 hours underconstant pressure. Following this final oligomerization, the compositewas removed from the mold and separated from the graphite foil. Thecomposite was then placed in a mullite tube furnace which was backfilledwith dry nitrogen. The furnace was ramped at 10° C./min to a finalpyrolisis temperature of 1200° C., held for two hours, and then cooled.The processing schedule, shown in Table 1, describes the temperaturesand the pressures applied. From a single impregnation bulk densities inthe range of 1.3 to 1.55 g/cc were achieved using 40% V_(f) (fibervolume fraction) chopped pitch based carbon fibers. This process wascarried out three times to maximize the density to approximately 1.75g/cc. TABLE 1 Temperature ° C. Pressure (MPa) Hold Time (hrs) 70 2.2 4890 4.4  1° C./min ramp 110 13 130 15 150 22 350 22 20 1200 — 10° C./minramp,  2 hr hold

During the reimpregnations a similar temperature-pressure process wasused with one additional step prior to hot pressing. This involvedplacing the composite in a steel pressure vessel and heating undervacuum for 30 minutes, backfilling with nitrogen and then introducingenough borazine oligomer to submerge the composite. The system was thenheld at 70° C. for 12 hours, with occasional venting to reduce thepartial pressure of hydrogen. This allowed for better permeation of thecomposite and caused the borazine oligomer molecular weight to increasewhile filling in the porous structure. As an example, one compositeexhibited an initial density of 1.46 g/cc and one, two and threeimpregnations yielded increased densities of 1.62, 1.66 and 1.75 g/cc,respectively.

3) X-ray Diffraction Data of C/BN Composites

The X-ray diffraction data depicted in FIG. 2 shows that the interlayer,or d-spacing, of the BN in the C/BN composites is 3.36 Å. It is knownthat hexagonal BN with d-spacings below 3.38 Å display greatly increasedresistance to hydrolysis (C. G. Cofer and J. Economy, “Oxidative andhydrolytic stability of boron nitride”, Carbon, Vol. 33, No. 4, p. 389).The BN d-spacing is calculated from the sharp peak labeled on the scan,appearing at 26.435°.

4) Wear Rate Testing of High Density C/BN Composites

Once the C/BN composites reached a density of approximately 1.75 g/cc,friction and wear testing was performed on an inertial brakedynamometer. This is the most important parameter since it determinesthe lifetime and safety of the brake material. The resulting wear rateplotted versus energy level for previously tested C/C, C/C/BN, and thenew C/BN composites of the invention appear in FIG. 3. The shapes of thecurves for the C/C and C/C/BN composites are similar, though C/C/BNexhibits a wear rate lower than the C/C at all levels. C/BN exhibits awear rate significantly lower than the C/C in the two problematicregions, namely low energy levels (taxi conditions) and high energylevels (Rejected Takeoff). A significant amount of wear over thelifetime of a C/C aircraft brake occurs during taxiing. In this regionthe C/BN displays a four-fold decrease in wear-rate, implying thepotential for a significant increase in the number of landings betweenoverhauls. At an energy level of approximately 600 kJ/kg the wear rateof the C/C material begins to increase. This increase in wear ratecorresponds to the onset of oxidation of the carbon. In comparison, theC/BN does not display significant oxidation until much highertemperatures (900-1000° C. for short times), which would correspond toenergy levels of approximately 1200 kJ/kg. Another advantage of thisincreased resistance to wear at high temperatures would be that theaddition of an expensive oxidative barrier coating (as used withcommercial C/C) would not be necessary.

5) Comparison with Traditional C/BN Materials.

The wear rate of the C/BN material of the invention is also markedlybetter than that of a traditional, lower density C/BN material producedaccording to the process of U.S. Pat. No. 5,399,377. As illustrated inFIG. 5, the wear rate of the C/BN of the invention is consistently below0.5 mg/m, whereas the traditional C/BN material exhibited wear ratesreaching above 2 mg/m.

6) Friction Testing of High Density C/BN Composites

The average coefficient of friction (COF) is plotted versus energylevels in FIG. 4. The resulting COF for the C/BN composites was found tobe much less sensitive to energy, and therefore temperature, than C/Ccomposites. Typically, the COF for C/C composites varies widely from 0.1to 0.5 depending on the temperature. In contrast, the COF for the C/BNcomposites varied only from 0.22 to 0.32 in the energy range tested.Moreover, as illustrated in FIG. 6, the material of the inventionexhibited a higher COF than traditional, low density, C/BN materials.The stable COF data for the C/BN composites allows for much betterpredictability in designing the braking system and resolution of issuesfor variability in the feel of brakes to pilots. It has also been shownthat the COF of boron nitride is less sensitive to the presence of water(G. W. Rowe, Wear, vol. 3, page 274, 1960). This will also help inresolving the issue of morning sickness, by decreasing the change in theCOF as the brakes dry out, caused by heating, during use.

7) Preparation of High Density 3D C/BN Composites.

The procedure previously described for the C/BN composite of Example 1was applied on a 3D needled preform instead of the chopped carbonfibers. The preform had a 28% fiber volume and a bulk density of about0.45 g/cc. The fiber of the preform was polyacrylonitrile-derived, withan average fiber diameter of 9 microns and a density of about 1.78 g/cc.Samples of the preform were provided by the Goodrich Corporation(Brecksville, Ohio) or Albany International Techniweave (Rochester,N.H.). The process was repeated up to three times, yielding final 3DC/BN composites with a density of 1.63 g/cc to 1.72 g/cc.

8) Preparation of High Density 3D C/C/BN Composites.

A sample of 3D needled preform such as that used in Example 7 was carbonvapor infiltrated to increase density to ˜1.3 g/cc, then placed in asteel pressure vessel and heated under roughing vacuum for 12 hours. Thepressure vessel was backfilled with dry nitrogen and borazine oligomerwas added. The preform and borazine were reacted for an additional 12hours in the steel pressure vessel. In a dry box, the borazine soakedsample was placed in a mold and an excess of borazine oligomer wasadded. The rest of the procedure then followed the same steps aspreviously described for the C/BN composite of Example 1. This processwas typically carried out four times, yielding a product with a densityof 1.62 g/cc to 1.80 g/cc.

9) X-ray Diffraction Data of High Density 3D C/C/BN Composites.

As seen in FIG. 7, X-ray diffraction scans of the high density 3D C/C/BNcomposite material displayed a highly ordered boron nitride phase withan interlayer spacing of 3.38 Å. The more defined shoulder appearingnext to the boron nitride peak is due to the vapor deposited carbonmatrix that tends to order itself on the surface of the carbon fiber.

10) Wear Rate Testing of High Density 3D C/C/BN Composites.

As seen in FIG. 8 and FIG. 9, the 3D C/C/BN systems displayed a wearrate so low that it is, at best, very difficult to measure accurately.The change in mass of the samples is very small, on the order ofmicrograms. The wear rate of the 3D C/C/BN is a full order of magnitudelower than the wear rates of 3D C/C up to an energy level ofapproximately 600 kJ/kg. At approximately 900 kJ/kg the wear rate of the3D C/C/BN composites starts to increase gradually due to oxidation butonly at higher interfacial pressures. In particular, the wear rate wasnear zero at low energy levels (<300 kJ/kg) that represent the taxiingcondition of aircraft braking. This regime accounts for the majority ofwear over the life of the brakes and may be significantly decreased withthe 3D C/C/BN composite. For example, the Boeing 777, designed for 3000landings per overhaul (LPO), only realizes 1500 LPO due to extra taxiingcaused by airport congestion. This wear would be essentially eliminatedwith the use of the 3D C/C/BN material. The samples were tested atinterfacial pressures of 0.25 MPa and 0.5 MPa, and the wear rate onlymarginally increased as the interfacial pressures was raised, as opposedto the more pronounced increases registered with 3D C/C and 3D C/BNmaterials.

11) Friction Testing of High Density 3D C/C/BN Composites.

At an interfacial pressure of 0.25 MPa, the 3D C/C/BN displays a COF ofapproximately 0.2 (FIG. 10). However, increased pressure leads to arelative decrease in the COF that is less substantial than thatregistered on 3D C/C and 3D C/BN materials (FIG. 11). It is interestingto note that at lower interfacial pressures and high energy levels, theCOF actually increases. This property would be especially important inthe case of an RTO where a higher COF is needed to affect decreased stoptime and distance, thus contributing to passenger safety.

1. A method of manufacturing a composite material comprising: forming amixture comprising a plurality of fibers and a borazine oligomer;subjecting the mixture to a first heating, for 12 hours to 56 hours; andsubjecting the mixture to a second heating; wherein the temperature ofthe first heating is 60° C. to 80° C., and the pressure during the firstheating is at least 0.5 MPa, the temperature of the second heating is atmost 400° C., and the greatest pressure of the second heating is atleast 15 MPa.
 2. The method of claim 1, further comprising subjectingthe mixture to a third heating, wherein the temperature of the thirdheating is at least 1200° C.
 3. The method of claim 1, wherein theborazine oligomer is obtained by heating borazine for 24 to 48 hours, ata temperature of 60° C. to 80° C.
 4. The method of claim 1, wherein thefibers are carbon fibers.
 5. The method of claim 1, wherein the pressureduring the first heating is 1 MPa to 6 MPa.
 6. The method of claim 1,wherein the temperature of the first heating is 65° C. to 75° C., andthe pressure during the first heating is 1.5 MPa to 5 MPa.
 7. The methodof claim 1, wherein the temperature of the first heating is 68° C. to72° C., and the pressure during the first heating is 2.0 MPa to 4.6 MPa.8. The method of claim 1, wherein the temperature of the second heatingis increased at a rate of 0.25° C./min to 3° C./min.
 9. The method ofclaim 1, wherein the temperature of the second heating is increased at arate of 0.75° C./min to 1.25° C./min.
 10. The method of claim 1, whereinthe temperature of the second heating is increased at a rate of 0.9°C./min to 1.1° C./min.
 11. The method of claim 1, wherein the greatesttemperature reached during the second heating is 130° C. to 170° C., andthe greatest pressure is 12 MPa to 32 MPa.
 12. The method of claim 1,wherein the greatest temperature reached during the second heating is140° C. to 160° C., and the greatest pressure is 16 MPa to 26 MPa. 13.The method of claim 1, wherein the greatest temperature reached duringthe second heating is 148° C. to 152° C., and the greatest pressure is21 MPa to 23 MPa.
 14. The composite material made according to themethod of claim
 1. 15. The composite material made according to themethod of claim
 2. 16. The composite material made according to themethod of claim
 3. 17. A composite material comprising carbon fibers ina boron nitride matrix, wherein the composite material has a density ofat least 1.62 g/cc.
 18. The composite material of claim 17, wherein thecomposite material has a density of 1.62 to 1.75 g/cc.
 19. A compositematerial comprising carbon fibers in a boron nitride matrix, wherein thecomposite material has a wear rate of at most 0.4 mg/m at an energylevel of 100 kJ/kg to 1100 kJ/kg, and a coefficient of friction of atleast 0.22 at an energy level of 100 kJ/kg to 1200 kJ/kg.
 20. A methodof manufacturing a composite material comprising boron nitride,comprising: forming a mixture comprising a preform and a borazineoligomer; subjecting the mixture to a first heating, for 12 hours to 56hours; and subjecting the mixture to a second heating; wherein thetemperature of the first heating is 60° C. to 80° C., and the pressureof the first heating is at least 0.5 MPa, and the temperature of thesecond heating is at most 400° C., and the greatest pressure of thesecond heating is at least 15 MPa.
 21. The method of claim 20, furthercomprising subjecting the mixture to a third heating, wherein thetemperature of the third heating is at least 1200° C.
 22. The method ofclaim 20, wherein the borazine oligomer is obtained by heating borazinefor 24 to 48 hours, at a temperature of 60° C. to 80° C.
 23. The methodof claim 20, wherein the preform is a 3D needled carbon fiber preform.24. The method of claim 20, wherein the preform is a CVI-infiltrated 3Dneedled carbon fiber preform.
 25. The composite material made accordingto the method of claim
 20. 26. The composite material made according tothe method of claim
 21. 27. A composite material comprising a 3D needledcarbon fiber preform impregnated with boron nitride having a density ofat least 1.63 g/cc.
 28. The composite material of claim 27, having adensity of 1.63 g/cc to 1.72 g/cc.
 29. A composite material, comprisingCVI-infiltrated carbon fiber preform impregnated with boron nitridehaving a density of at least 1.62 g/ cc.
 30. The composite material ofclaim 29, having a density of 1.62 to 1.80 g/cc.
 31. A compositematerial comprising a 3D needled carbon fiber preform impregnated withboron nitride having a wear rate of at most 0.05 mg/m at an energy levelof 100 kJ/kg to 1000 kJ/kg, and a coefficient of friction of at least0.12 at an energy level of 100 kJ/kg to 900 kJ/kg.
 32. A brake foraircraft comprising the composite material of claim
 17. 33. A brake foraircraft comprising the composite material of claim
 27. 34. An aircraftcomprising the brake of claim
 32. 35. An aircraft comprising the brakeof claim
 33. 36. A method for decelerating an aircraft comprisingbraking the aircraft with the brake of claim
 32. 37. A method fordecelerating an aircraft comprising braking the aircraft with the brakeof claim 33.